1. Field of the Invention
The present invention relates to a large scale integrated circuit (LSI), and more specifically to a CMOS LSI current source switch circuit for switching on and off an electric current to be provided from the current source of the LSI to a load.
2. Description of the Related Art
Various circuits such as a voltage control oscillator (VCO), a loop filter used for a phase locked loop (PLL), a timer using a time constant of an RC, etc. can control the current source of an LSI.
FIGS. 1 and 2 show the conventional technology of a current source switch circuit for switching on and off the electric current flowing from a current source to a load.
In the circuit shown in FIG. 1, a FET 11 functions as a switch for turning on and off the electric current provided from a MOS FET (current source) 10 which is connected to, for example, the ground side, and forms part of a current source; and a load 12 connected to, for example, an power supply are serially connected. When it becomes unnecessary to provide an electric current for the load 12, the circuit shuts off the FET 11 according to a control signal input to the gate of the FET 11, and the current source 10 is disconnected from the load 12.
The circuit shown in FIG. 2 is often used in a high-speed digital/analog converter, etc. In the circuit shown in FIG. 2, a FET 13 is connected between a power supply and a node N1, that is, between the current source 10 and the switch 11. To the gate of the FET 13, an inverter 14 provided by inverting the control signal provided for the gate of the switch 11 is connected. In this circuit, when it is not necessary that an electric current flows through the load 12, the switch 11 is turned off according to the control signal, and the switch 13 is turned on. As a result, the electric current flowing through the current source 10 is returned to the power supply, that is, abandoned.
According to the first conventional technology shown in FIG. 1, the current source 10 is turned off by turning off the switch 11. Therefore, it takes some time to reach a necessary value of the node N1 to operate the current source 10. Furthermore, the amount of the shift of an electric charge is large in switching on/off compared to the junction capacity of the FET. As a result, there has been the problem that a quick switch cannot be realized.
To solve the above described problem, according to the second conventional technology, the current source 10 is not turned off, and the shift of the electric charge to the junction capacity is minimized to a successful quick switch. However, this technology has the problem that it requires more electric power to operate the current source 10 even when it is not necessary to provide an electric current for the load 12.